Overload protective relay



June 17, 1941. SCHRQDER 2,246,324

OVERLOAD PROTECTIVE RELAY Filed Sept. 7, 1939 2 v 1 4 F i JHTUEABLE I654070; l L im. F

WITNESSES. Mariam mvamo z 72752 Sc/zradar:

Patented June 17, 1941 UNlTED STATES TENT QFFECE OVEBLOAD PROTECTIVERELAY Application September 7, 1939, Serial No. 293,683 In GermanyNovember 29, 1938 12 Ciaims.

My invention relates to a protective system and particularly to anoverload protective system for electrical apparatus. In its morespecific aspects, my invention relates to a protective system in which arelay, such as a thermal bimetal relay, operates during overloads on theelectrical apparatus to be protected with a time delay dependent on themagnitude of the overload.

Electrical apparatus which is subject to overload has a very definitetime-overload curve from which may be determined how long a time adefinite overload may be maintained without exceeding the maximumpermissible heating in the apparatus. Likewise a definite curve may bedrawn showing the relationship between the amount of overload and thetime that overload must exist before an overload time-delay relayoperates to open the circuit. A complete protection of the apparatus isattained if the overload relay has an operating curve which preciselycorresponds with the overload curve of the apparatus. However, theadapting of the operating curve of the protective relay to the overloadcurve of the apparatus is frequently not possible to the desired extent.This applies in particular to power rectifier apparatus which, ingeneral, can carry relatively small overloads for comparatively longintervals of time but must be disconnected from higher overloads after avery short time.

It is an object of my invention to secure improved overload protectionby attaining closer correspondence between operating curve of theoverload relay and the overload curve of the apparatus to be protectedthan has heretofore been possible.

Other objects of my invention will become evident from the followingdetailed description taken in conjunction with the accompanying drawing,in which:

Figure 1 shows graphs of the overload curve of the apparatus to beprotected and the operating curve of a thermal protective relay of aknown type.

Fig. 2 shows a circuit arrangement exempiifying my invention, and

Fig. 3 shows curves in which variations of the potential across theshunt in the load circuit and the potential across the intermediatetransformer are plotted against variations in the load current.

The relationships previously discussed as. obtained by prior art devicesare reproduced in Fig. 1 of the drawing, wherein the overload isrepresented in percentage of rated power as a funcprimary winding of thetransformer rises.

tion of the permissible overload interval or operating time. In thisgraph, A represents the overload curve of a rectifier while R representsthe operating curve of a bimetal relay. It will be evident that thecurves A and R do not correspond closely enough to afiord completeprotection since the relay will open earlier than necessary for smalloverloads but too late to avoid injury to the apparatus in the case ofhigh overloads.

In Fig. 2 a thermal over-current relay I controls the operating circuitfor the trip device of breaker 2. The control circuit for the relay l issupplied through a current transformer 3, the primary coil of which isenergized from the secondary coil of an intermediate transformer 4, theprimary coil of which is energized, in turn, from an alternatingpotential source 5 which has approximately constant potential. In theprotection of a rectifier, for example, the alternating potential sourcemay be one phase of the alternating current network supplying therectifier.

In order to modify the voltage impressed upon the transformer i inproper relation to the varying load current, a saturable reactor orchoke coil 6 is provided, the voltage winding 1 of which is placed inseries circuit with the source 5 and the transformer 4. A premagnetizingor control winding 8 of this reactor is energized, in a pre- .ferredform, by current derived from a pair of shunt leads 9 from a meteringshunt or resistance i t placed in series with one main load line I i.

A quick acting relay i2 may be placed in series circuit with thesecondary coil of transformer 3 to close the circuit to the breaker 2independently to the thermal relay I upon the occurrence of anabnormally high overload.

The potential of the alternating current source 5 is distributed overthe coil 1 of the saturable reactor 6 and the primary Winding of thetransformer 4 in the ratio of their inductances. As long as the currentin the shunt iii, and accordingly the current in the magnetizing coil 8,is small, the inductance of the coil 1 is comparatively large so that alarge portion, or the largest portion, of the alternating potential fromthe source 5 is impressed across its terminals.

As long as the current in the winding 3 is insuificient to cause thecore of the reactor 4 to approach saturation, the potential across thewinding 1 changes very little. If, however, with increasing current, thechoke core approaches saturation, the inductance of the coil l begins todecrease more rapidly so that the potential on the If,

after this, the reactor core is fully saturated, the potential impressedacross the intermediate transformer 4 rises only a little with furtherincrease of .the load current in the shunt l0.-

In Fig. 3 the potential change in the transformer 4 is plotted in curveC as a function of the current through the shunt ID. The potential onthe shunt i is, on the other hand, represented by the straight line B.It will be noted that for the selected relationship that in the regionbetween approximately rated load and fifty percent overload, thepotential on the transformer .4 is less than, and does not vary inproportion to, the voltage across the shunt l0. Thus, between these loadvalues the potential, and consequently the current through the relay I,will be less than if they varied directly with the amount of overload.As a result, the time delay relay will require more time beforeoperating and the operating curve R of Figure 1 will be raised betweenthe points of zero and fifty percent overload to correspond more closelyto the overload curve A. For overloads above fifty percent the curves ofFig. 3 show that as a result of the reactance coil 6, the voltage oftransformer 4 is greater, and consequently the relay I will operateafter a shorter time interval, than that shown in Fig. 1. Thus theoperating curve R of Fig. 1 will be shifted to the left for values abovefifty percent overload to more closely correspond to overload curve A.

Of further advantage is the fact that the thermal relay, as curve C inFig. 3 also shows, is, in the region between zero load and the ratedload or current, supplied by a current which even in the zero loadcondition corresponds approximately to half the rated load. As a resultof this the thermal relay is always in a heated condition propitious foroperation so that inaccuracies in the release time, as a consequence ofvarying initial temperatures, are avoided.

It will be evident that the operating curve of the relay may be furtherchanged and adapted to other operating conditions by changing thecharacteristics of the premagnetizing winding 8.

Although I have shOWn and described certain specific embodiments of myinvention, 13 am fully aware that modifications thereof are possible. Myinvention, therefore, is not to be restricted except insofar as isnecessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a protective system, a load circuit, a circuit interrupterinterposed therein, voltage controlled delay means for effectingoperation of said circuit interrupter, a magnetizable core, a reactancecoil on'said core, a magnetizing coil on said core and means connectingit for energization from said load circuit in proportion to the avarying current therein, a constant voltage supply circuit and meansconnecting it in series circuit with said reactance coil, and meansconnecting said voltage controlled delay means for energization fromsaid series circuit.

2. In a protective system for a load circuit, a

main'breaker, relay means to control the operation of said breaker,voltage controlled delay means for effecting operation of said relaymeans, a ma netizable core, a reactance coil on said core,

a magnetizing coil on said core and means forenergizing it from saidload circuit in proportion to the varying current therein, a constantvoltage supply circuit, means connecting said reactance coilin seriescircuit with said constant Voltage supply circuit, and means connectingsaid voltage ,ing, means connecting said reactance coil and controlled.delay means for energization from said series circuit.

3. A protective system according to claim 2, wherein the voltagecontrolled delay means comprises a thermal element.

4. A protective system according to claim 2, wherein is provided asecond relay means and means connecting it for energization from saidseries circuit to control the operation of said breaker upon theoccurrence of high overloads in said load circuit, said second relaymeans being operable to open the main breaker prior to operation of thefirst named delayed relay upon the occurrence of abnormal circuitloading.

5. In a protective system for a direct current load circuit, a mainbreaker, relay means to control the operation of said breaker, voltagecontrolled thermal delay means for eifecting operation of said relaymeans, a magnetizable core, a reactance coil on said core, a magnetizingcoil on said core, a shunt in said load circuit, means for energizingsaid magnetizing coil from said shunt in proportion to the varyingcurrent in said load circuit, a constant voltage supply circuit andmeans connecting it in series circuit with said reactance coil, andmeans connecting said voltage controlled thermal delay means forenergization from said series circuit.

6. In a protective system for a load circuit, a main breaker, relaymeans to control the operation of said breaker, voltage controlledthermal delay means for effecting operation of said relay means, amagnetizable core, a reactance coil on said core, a magnetizing coil onsaid core and means for energizing it from said load circuit inproportion to the varying current therein, a constant voltage supplycircuit, a transformer including a primary winding and a secondarywinding, means connecting said reactance coil and said primary windingin series circuit with said constant voltage supply circuit, and meansconnecting said voltage controlled thermal delay means for energizationfrom said secondary Winding.

'7. In a protective system for a direct current load circuit, a mainbreaker, relay means to control the operation of said breaker, voltagecontrolled thermal delay means for eifecting operation of said relaymeans, a magnetically saturable core, a reactance coil on said core, amagnetizing coil on said core means for energizing it in proportion tothe varying current in said load circuit, a constant voltage supplycircuit,

and a transformer including a primary windingload circuit, a mainbreaker, relay means to con trol the operation of said breaker, voltagecontrolled thermal delay means for eifecting operation of saidrelaymeans, a magnetically saturable core, a reactance coil on saidcore, a magnetizing coil on said core, a shunt in said load circuit,means connecting said magnetizing coil for energization from said shuntin proportion to the varying current in said load circuit, a C011".stant voltage supply circuit, a transformer including a primary windingand a secondary Windsaid primary winding in series circuit with saidconstant voltage supply circuit, means connectsecondary ing said voltagecontrolled thermal delay means for energization from said secondarywinding, a second relay means and means connecting it for energizationfrom said secondary Winding to control the operation of said breakerprior to the operation of the thermal delay means upon the occurrence ofabnormally high overloads in said load circuit.

9. In a protective system for a load circuit, a main breaker, relaymeans to control the operation of said breaker, voltage controlledthermal delay means for effecting operation of said relay means, amagnetically saturable core, a reactance coil on said core, amagnetizing coil on said core, and means for energizing it from saidload circuit in proportion to the varying current therein, a constantvoltage supply circuit, means connecting said reactance coil in seriescircuit with said constant voltage supply circuit, and means con nectingsaid voltage controlled delay means for energization from said seriescircuit, said magnetically saturable core and said magnetizing coilbeing so dimensioned as compared to the imped ances in said seriescircuit that the current acting on said delay means betweenapproximately zero and fifty percent overload on said load circuit issmaller than the current linearly dependent on said overload current andfor values of said overload current above fifty percent is larger thanthe current linearly dependent on said overload current.

10. In a protective system for a load circuit, a main breaker, relaymeans to control the operation of said breaker, voltage controlled delaymeans for eflecting operation of said relay means, a magnetizable core,a reactance coil on said core, a magnetizing coil on said core and meansfor energizing it from said load circuit in proportion to the varyingcurrent therein, and a constant voltage supply circuit, means connectingsaid reactance coil in series circuit with said constant voltage supplycircuit, and means connecting said voltage controlled delay means forenergization from said series circuit, said magnetizable core beingdesigned to be saturated by the current in said magnetizing coil whenthe overload in said load circuit is greater than one hundred percent.

11. In combination with a protective system according to claim 9, asecond relay means and means for connecting it for energization fromsaid series circuit to eifect operation of said breaker immediately uponthe overload current in said load circuit reaching approximately onehundred percent.

12. In a protective system for a load circuit, a main breaker, relaymeans to control the operation of said breaker, voltage controlled delaymeans for eifecting operation of said relay means, a magnetizable core,a reactance coil on said core, a magnetizing coil on said core and meansfor energizing it from said load circuit in proportion to the varyingcurrent therein, a constant voltage sup-ply circuit and means connectingit in series circuit with said reactance coil, and means connecting saidvoltage controlled delay means for energization from said seriescircuit, said magnetizable core being so designed that the steepestportion of its saturation curve occurs approximately between full loadcurrent and one hundred percent overload current in said load circuit.

ERNST scHRoDER.

